The present invention is generally directed to BALUN transformer circuits for analog signals. More particularly, the present invention is directed to the construction of BALUN circuits which are formed on printed circuit boards or other insulative substrates. Even more particularly the present invention is directed to BALUN transformer structures in which high and low impedances are realized without the use of twisted wire pairs or the use of a ferrite core.
A BALUN is an electronic circuit component which is employed to provide impedance balance in radio frequency (RF) circuits. In essence the BALUN is a balanced to unbalance transformer. These devices are used in radio frequency applications to connect unbalanced components and circuits to balanced ones.
Unbalanced circuit components are usually large and expensive and include such devices as filters and power amplifiers. To make them balanced, while possible, would require them to be twice as large and twice as expensive. On the other hand, balanced components are usually found in integrated circuits because the cost of doubling the number of internal components is low in an integrated circuit structure. Balanced circuits also have desirable electrical properties. They have improved signal isolation and linearity characteristics and accordingly are used whenever possible. In an unbalanced circuit the current is typically carried by a single wire and returns through a ground which is preferably a large conductor which either forms the chassis for the device or is otherwise directly and electrically connected directly to the chassis. On the other hand, a balanced circuit typically includes two conductors and a ground. In the BALUN case, current flows "down" one conductor and returns via the other one. This eliminates current flowing through the ground. This is the ideal situation.
BALUN circuits are desired in radio design because chip designs tend to be differential while external components tend to be single ended. Hence a BALUN transformer is needed to couple internal and external components. However, BALUNS generally have limitations with respect to cost and size particularly when it comes to their use in radio frequency devices such as cellular telephones. These devices clearly require BALUNS that exhibit small sizes particularly with respect to the other components in a cellular telephone or cellular telephone system. Hence there is a need for small inexpensive BALUN transformers that also exhibit good electrical performance. Additionally, while it is possible to implement BALUNS as an integrated circuit, such circuits have poor electrical performance and typically exhibit high loss.